Why spy on allies? Even good friends keep secrets

"I am persuaded that everyone knew everything or suspected everything," Russian Foreign Minister Sergey Lavrov said of the reports of U.S. monitoring.

And while prime ministers and lawmakers across Europe and Asia say they are outraged, Clapper told Congress that other countries' own spy agencies helped the NSA collect data on millions of phone calls as part of cooperative counterterror agreements.

Robert Eatinger, the CIA's senior deputy general counsel, told an American Bar Association conference on Thursday that European spy services have stayed quiet throughout the recent controversy because they also spy on the U.S.

"The services have an understanding," Eatinger said. "That's why there wasn't the hue and cry from them."

And another intelligence counsel says the White House can reasonably deny it knows everything about the U.S. spying that's going on.

"We don't reveal to the president or the intelligence committees all of the human sources we are recruiting. ... They understand what the programs are, and the president and chairs of the intelligence committees both knew we were seeking information about leadership intentions," said Robert Litt, general counsel for the Office of the Director of National Intelligence. "They both saw reporting indicating what we were getting if not indicating the source."

Still, Claude Moraes, a British Labor Party politician and member of the European Union delegation that traveled to Washington this week for talks about U.S. surveillance, was troubled by the broad net being cast by U.S. intelligence.

"Friend-upon-friend spying is not something that is easily tolerable if it doesn't have a clear purpose," he said. "There needs to be some kind of justification. ... There is also a question of proportionality and scale."

Obama has promised a review of U.S. intelligence efforts in other countries, an idea that has attracted bipartisan support in Congress.

The United States already has a written intelligence-sharing agreement with Canada, Britain, Australia and New Zealand known as "Five Eyes," and France and Germany might be interested in a similar arrangement.

Paul Pillar, a professor at Georgetown University and former CIA official, worries that a backlash "runs the risk of restrictions leaving the United States more blind than it otherwise would have been" to overseas developments.

The effort to strike the right balance between surveillance and privacy is hardly new.

University of Notre Dame political science professor Michael Desch, an expert on international security and American foreign and defense policies, says the ambivalence is epitomized by Secretary of State Henry L. Stimson's famous line, "Gentlemen do not read each other's mail." Stimson, who served under President Herbert Hoover, shut down the State Department's cryptanalytic office in 1929.

"Leaks about NSA surveillance of even friendly countries such as Brazil, Mexico, and now France make clear that we no longer share Stimson's reticence on this score," Desch said. "While such revelations are a public relations embarrassment, they also reflect the reality that in this day in age, gentlemen do read each other's mail all of the time, even when they are allies."

In fact, a database maintained by the Defense Personnel Security Research Center covering Americans who committed espionage against the U.S. includes activity on behalf of a wide swath of neutral or allied countries since the late 1940s. U.S. citizens have been arrested for conducting espionage on behalf of South Korea, Taiwan, the Philippines, Israel, the Netherlands, Greece, Saudi Arabia, Egypt, Iraq, Jordan, Ghana, Liberia, South Africa, El Salvador and Ecuador, according to the database.